PARIS (Reuters) - French investigators trying to solve the murder of three Kurdish activists in Paris have collected evidence about the chief suspect's connections to Turkey, four sources with knowledge of the investigation told Reuters.

Police sources told Reuters the magistrate in charge of the case was about to lodge a formal appeal for information to Turkey about Omer Guney, a Turkish immigrant placed under formal investigation for the triple murder eight months ago.

The move could mark a turning point in the case, which has become a rallying cry for Europe's large population of ethnic Kurds. It comes after disclosures that Guney took at least three trips to Turkey and made dozens of phone calls to contacts there in the months before the killings, lawyers with access to investigation files told Reuters.

The Turkish justice ministry did not immediately respond to requests for comment on cooperation with France in the case.

The murders of Sakine Cansiz, 55, a founding member of the Kurdistan Workers Party (PKK); Fidan Dogan, 32, a spokeswoman for the cause in France and Europe; and a trainee named Leyla Saylemez, 25, sent a shockwave through Europe's large Kurdish community. The women were shot as ceasefire talks to end 29 years of war between the PKK and Turkey were starting.

The key question asked by lawyers and victims' family members is who ordered the killing. Kurds who gather each week by the crime scene say it was a political assassination.

French police quickly arrested Guney, 30. Surveillance footage placed him at the scene, and partial DNA from one of the victims was found on a parka belonging to him, lawyers said.

Guney, who says he is innocent, has been awaiting trial for eight months in solitary confinement near Paris. His lawyer, Anne-Sophie Laguens, said she planned to apply to have him freed under court supervision because he was not receiving proper treatment for a brain tumor that induced seizures.

Laguens said she was also waiting for answers from Turkey regarding her client's trips. Guney told investigators he had travelled to Turkey to find a wife and had bought tickets with disability payments he received from the French state.

POLITICAL FALLOUT

Lawyers both for Guney and the victims' families in France and in Turkey say the investigation has dragged due to concern about political fallout from a case involving two NATO allies linked by a 2011 bilateral security accord.

"It's my impression that we (the French investigation) have received more information in this case through Turkish media than through international cooperation," said Antoine Comte, a lawyer for the Kurdish victims in France.

Police sources said Turkish authorities had earlier provided some biographical information about Guney, but the French magistrate was expected to seek responses to recent disclosures.

A spokesman for France's foreign ministry said the French state exerts no influence over judicial investigations. Paris' anti-terrorism court denied that political tension was slowing down the case.

New evidence could upset a ceasefire brokered between the PKK and Turkey: Kurdish rebels are disappointed with Turkish efforts to address their grievances and have said they are considering whether to maintain the deal.

Lawyers also questioned the efficiency of judicial cooperation after the Turkish pro-government newspaper Bugun wrote that the prosecutor in Ankara had accused French authorities in August of failing to respond to his requests for details in the case.

Turkish media wrote earlier this year that the Ankara prosecutor is conducting a separate probe under an article of penal law which says a person who commits a crime abroad while in the service of the Turkish state can be tried in Turkey, even if he is already found guilty abroad and/or has served time.

Turkish media said the Ankara prosecutor is seeking to establish whether Guney was in the service of the Turkish state. The prosecutor's office did not respond to requests for comment.

"We feel that since the crime was committed in France, the real interlocutors are the French authorities. They must respond to the Turkish requests for information," said Meral Danis Bestas, a lawyer in Turkey for the victims' families.

Two pieces of evidence in investigation files highlight Guney's alleged ties to people in Turkey: three trips in August, October and December of 2012, and phone records from one of five cell phones that police say belonged to Guney. The latter show "dozens" of calls to Turkish numbers in the same period.

PHONE RECORDS

Comte said records of Guney's phone activity with Turkey were placed in the investigations file in July, five months after his arrest. These contacts could be crucial to finding out whether Guney was involved in the killings and, if so, with or without foreign backing. However, the details cannot be checked without help from Turkey, Comte said.

"You need an order from a Turkish judge to identify the interlocutors," said another lawyer for the victims' families, Jean-Louis Malterre.

In France lawyers for victims can join criminal proceedings. They have access to investigation files and participate in trials. The Turkish system has similar provisions.

While the French magistrate prepares to seek information from Turkey, one of the lawyers with access to the investigation file pointed also to hold-ups on the French side.

A month after Guney's arrest, investigators from the French anti-terrorist unit, Sdat, checked the contents of a borrowed Peugeot car he used on the day of the killing; it was their second try. Dismantling the car, they found a passport behind the radio with stamps for three trips to Turkey, and a dry-cleaning bill dated a few days after the killings, Comte said.

"When Guney was brought in, they missed half the things in his car," the lawyer said. "The dry-cleaning bill didn't enter the investigation file until a month later. If you look at the transcripts of the first hours of questioning, all they are doing is trying to update their archives about PKK activities."

Police sources had no comment on allegations that evidence was missed in the first search of Guney's car. They said questioning had focused on his links to the PKK because he claimed to be a member. PKK officials have denied Guney was a member of the group.

The appeal to Turkey for judicial help, to be lodged by investigating magistrate Jeanne Duye, comes after similar requests were sent to Holland and Germany - where Guney lived for nine years - and received replies.

Other factors are also complicating the investigation. On September 25 Duye's computer containing judicial files was stolen from her home. Duye's office did not respond to a request for comment. Duye has not spoken publicly about the murder case.

(Additional reporting by Ayla Jean Yackley in Istanbul and Nicolas Bertin in Paris; Editing by Richard Woods and Giles Elgood)